According to this FAQ:

Q: Can I see subtasks on the list?
A: Yes, you can. An identifier is shown on the task bar if there are subtasks in the task. Click it and you'll see the subtasks and you can also complete them by checking the checkbox in front of them.

...i should see the identifier in all clients? I do see it on the Mac and Windows clients, but not in the Web. Am I missing something?

PS: It's not on the iOS client either, but a popup window would be a but cludgy there, I guess. A visual clue that there are subtasks would still be nice.